French Toast is amazing. We have all had many versions and some are better than others. The origin of this dish was to use up some stale bread. It really needs to have a hearty, almost dried out bread to make great french toast. If you have ever use fresh white bread you will know that bread matters! I love sourdough for making french toast. The flavor and texture really work great for this dish. The better the bread the better the results but you do really want to use something that has been around for a few days. I have found a great sourdough called Goldminer Square California that is available at my local market and should be available at many Kroger stores around the country. Pretty much all bread is baked locally and will vary from place to place. If you have a great local bread, use it. Just make sure it is "real" sourdough and not some chemical flavoring in there. They do not hold up well when soaked in the egg mixture. The Francisco brand is a more commercial sourdough and I have also used it with good success.
Size and shape of bread? Sourdough is a process not a style. You will find small, large, round, thick and thin. Some are made in pans and others are free form. I like a larger slice and if you can get thicker slices even better! Up to about 1" works great. You want the egg mixture to be able to soak in so any thicker gets tricky. The size and thickness of the bread will also affect the number of eggs and amount of milk you need. 1 Egg per slice of bread is not a bad rule but it may 1 1/2 eggs for some breads. Good news is this does keep for several days in the refrigerator or up to 2 months in the freezer if you make too much.
Now, lets talk about cinnamon and vanilla. These are great in most french toast but the thing I love about sourdough is its tartness. It has a much more savory flavor than other bread so why cover it up with a bunch of other stuff. I do add a touch of sugar but that is more about building a solid custard with the egg/milk mixture. The sugar molecules help build that great texture that makes this french toast extra special. It does mean you have to whip the eggs for several minutes until the color starts to get get very light. You can use a hand whisk but a hand mixer is MUCH easier!